1. Field of the Invention
The present invention relates to a thermal head for use in a thermal recording apparatus such as a facsimile, a printer, or the like, and a method of manufacturing the same.
2. Description of the Prior Art
Recently a thermal recording system has been introduced in large quantities, because of its good printing quality and inexpensive running cost. Generally, there are two types of thermal heads which are the core portion of the thermal recording system: one is a line type thermal head having a heating portion provided with heating dots alined along the entire transversal length of a printing paper; and the other is a serial type thermal head having a heating portion provided with heating dots longitudinally alined over the length of one character to be printed. According to the extension of the thermo-sensitive system, the development of inexpensive thermal heads with higher resolution has been demanded. Today, there are three types of thermal heads, that is a thick film type, a thin film type, and a semi-conductor type. The thin film type thermal head has become the main current as the demand of the high resolution described above become strong. As for cost down, since the above-mentioned three types of thermal heads utilize a sputtering or evaporation method to form a film structure it is strongly desired to efficiently form a film structure in order to realize cost down.
Each of the conventional thermal heads of such a serial head type as shown in FIG. 7 and of such a line head type as shown in FIGS. 8 and 9 has a pattern constituted by a heating portion 6 for actually performing printing and a lead wire portion 7 for connecting the heating portion 6 with the outside. As shown in FIG. 10, the heating portion 6 is constituted by: an undercoating film 13 formed on an electrically insulating substrate 1 of such as ceramics, glass, or the like, for improving tightness between the substrate 1 and the construction film of the head; a heating resistor film 2; a conductor film 3; an oxidation resisting film 4 for the heating resistor 2; and a wear resisting film 5 for protecting the head, these films 2, 3, 4, and 5 being layered one on one in the order described. As the substrate used in a thermal head, such a substrate having a glass glaze 14 as shown in FIG. 11 may be employed in order to improve tightness as well as thermal response between the heating portion 6 and the printing paper. The two systems described above, however, have the same film structure of thermal head and the conductor film 3 is formed on the heating resistor film 2.
As the method of producing the film type thermal heads, such a method is employed in which a plurality of thermal heads are produced at a time from a single sheet of insulator substrate 1 made of ceramics, glass, or the like, and which has the steps of forming the undercoating film 13 all over the surface of the insulator substrate 1, forming the heating resistor film 2 by printing, spattering, or evaporation, forming the conductor film 3 on the resistor film 2, and forming a predetermined head pattern by using the photo-lithographic technique. Then the oxidation resisting film 4 for the heating portion and the wear resisting film 5 are formed by spattering, or the like, to cover the head pattern throughout the substrate 1 to complete the heads. Upon completion, the heads are divided into individual ones along snap lines put into the substrate 1 beforehand or divide the substrate 1 by using a die device.
As described above, since the conventional thermal head has a construction formed in such a manner that a plurality of thermal heads each having the heating portion 6 and the lead wire portion on the same substrate are produced by the same steps, the oxidation resisting film 4 for protecting the heating resistor film 2 of the heating portion 6 and the wear resisting film 5 are formed not only on the heating portion 6 but also on the lead wire portion 7 which is unnecessary to be protected and which occupies a most part of the area of the thermal head. The structure is therefore useless in film forming. Further, there is such irrationality that the number of the heads to be produced from one sheet of the substrate is limited not by the size of the heating portion 6 but by the size of the lead wire portion 7 because the heating portion 6 and the lead wire portion 7 are constituted on the same substrate as described above. Further more, there is such a disadvantage that there is a risk of damage such as crack or distortion even in the heating portion 6 of each thermal head when the substrate is divided into individual thermal heads because the division of performed by using physical force such as by applying external force along the snap lines on the substrate or by using die device.